1. Field of the Invention
The present invention relates to a thermosetting polyester-type powder coating composition containing, as a vehicle a poly-condensate obtained from 2-methylbutane-1,3-diol (isopentyl glycol) (hereinafter referred to as "IPG", for simplicity) and terephthalic acid (hereinafter referred to as "TA", for simplicity) or dimethyl terephthalate (hereinafter referred to "DMT", for simplicity) as an essential ingredient.
2. The Description of the Prior Art
Recently, a powder coating which does not contain an organic solvent which causes air pollution, has been strongly desired from a social standpoint, and such a coating which does not use an organic solvent has been seriously considered. Various materials such as an acrylic resin, an epoxy resin or the like have been proposed as a vehicle of a powder coating. However, disadvantages exist because an acrylic resin type powder coating is expensive and has poor chemical resistance. Further the inherently bad smell of the acrylic acid is generated on curing. An epoxy resin type powder coating has poor weatherability and ultraviolet light resistance.
Recently a polyester type powder coating composition has been rapidly investigated and various reports thereon have been published. However, use of IPG as an essential polyol component has not been reported, and reports merely have been that a polyol other than IPG can be used. For example, a vehicle for a powder coating comprising (a) 70 to 99% by weight of a polycondensate obtained from 40 to 55 mole % of aromatic dicarboxylic acids (e.g., TA or DMT), 15 to 50 mole % of hydrogenated bisphenols and 5 to 40 mole % of aliphatic diols (e.g., ethylene glycol, propylene glycol or butylene glycol) and (b) 1 to 30% by weight of an amino resin is described in Japanese Patent Publication No. 6615/73. However, this conventional polyester type powder coating is not sufficiently satisfactory from the standpoint of storage stability and with respect to various properties such as smoothness, gloss, adhesiveness, boiling water resistance, etc. of a film obtained therefrom (see Comparative Example 2 described hereinafter).
Generally, it is desired for a powder coating to have sufficient storage stability (or powder stability) that blocking (or caking) after the production thereof does not occur. To achieve the above purpose, the softening point of the base resin must be high. On the other hand, since the flow properties of a powder coating on coating or curing must be good, the softening point of the base resin should be low. Thus, opposing requirements must be satisfied. Further, it is desirable for the mixing with a pigment to be easy in preparing a powder coating and pulverizing and screening of the powder coating desirably, should also be easy. The flowability on curing of a powder coating thus prepared preferably should also be good, curing to form a film should also occur at a temperature which is, relatively, not so high and pinholes, blisters, etc. should do not occur. Thus, coating should have a good smoothness, gloss, flexibility, chemical resistance, stain resistance and water resistance.